1. Field of the Invention
The present invention relates to an organic EL element drive circuit and an organic EL display device using the same drive circuit and, in particular, to an organic EL display device for use in an electronic device such as a portable telephone set or a PHS, etc., which can provide a wide dynamic range in regulating a drive current and is capable of absorbing luminance difference due to difference of organic EL light emitting material and which is suitable for high luminance color display.
2. Prior Art Description
An organic EL display panel of an organic EL display device to be mounted on a portable telephone set, a PHS, a DVD player or a PDA (Personal Digital Assistance), etc., having 396 (132×3) column line terminal pins and 162 row line terminal pins has been proposed and the number of the column line terminal pins as well as the row line terminal pins tends to be still increasing.
An output stage of a current drive circuit of such organic EL display panel includes current source drive circuits, for example, output circuits using current mirror circuits, provided correspondingly to the terminal pins regardless of the type thereof, i.e., active matrix type or passive matrix type.
The problems of the organic EL display device are that, if a voltage drive is used as in the liquid crystal display device, variation of luminance becomes large and that a display control becomes difficult due to difference in luminance between red, green and blue color. Therefore, ratio of light emitting efficiency for red (R), green (G) and blue (B) colors becomes, for example, R:G:B=6:11:10 even if the current drive is used. In addition, the light emitting efficiency depends upon material of the organic EL element to be used.
Therefore, in order to obtain white balance on a display screen, the current drive circuits for a color display include drive current regulator circuits for regulating luminance of R, G and B colors correspondingly to the organic EL materials to be used.
Since, in the current drive circuit of the organic EL display device, the drive current for each color is usually generated for each of the respective column pins by amplifying a reference current, the regulation of drive current for obtaining white balance is performed by regulating the reference currents corresponding to red, green and blue colors.
It has been usual that, in order to regulate the reference currents, a reference current generator of the conventional drive current regulation circuit, which is provided for each of R, G and B, includes a D/A converter circuit of about 4 bits and the reference current is regulated by setting a predetermined bit data within a range from 30 μA to 75 μA with step of, for example, 5 μA. However, since various organic EL materials have been developed, the luminance regulation range of the 4-bit D/A inverter circuit is insufficient.
If, in order to solve this problem, the number of bits of the D/A inverter circuit is increased to 6 to 8, the circuit size of the drive current regulator circuit becomes substantial, so that it becomes difficult to provide the current drive circuit as one chip, causing miniaturization of the display device to be impossible.
On the other hand, there is a recent request of a dynamic range of reference current regulation within a range from 0 μA to 75 μA with step of 1 μA.